2-Oxytetrahydrofurans are useful intermediates for the synthesis of 2,3-dihydrofuran, a monomer that polymerizes to give degradable polymers (see, for example, JP Kokai Nos. 03-192,116 [Chem. Abstr. 115 256951h], 02-11,622 [Chem. Abstr. 112 236049e], and 03-146,509 [Chem. Abstr. 115 209876c], and European Patent Application No. 434,067 [Chem. Abstr. 115 233814b]). 2-Oxytetrahydrofurans are also useful in the pharmaceutical area for the synthesis of tetrahydrofuryl-substituted fluorouracils (Heterocycles 6 (1977) 529 [Chem. Abstr. 87 23202c]; JP Kokai No. 56-015284 [Chem. Abstr. 95 7323c]; JP Kokai No. 56-015285 [Chem. Abstr. 95 7322b]; JP Kokai No. 52-093778 [Chem. Abstr. 88 121226d]).
Synthetic routes to 2-oxytetrahydrofurans are known, but most require expensive reagents or starting materials, give low yields, or involve complicated processes.
Japanese Patent Application Kokai No. 52-106860 describes a single-phase process for producing 2-oxytetrahydrofurans from anhydrous 2-hydroxytetrahydrofuran using an acid catalyst. The reference teaches that 2-hydroxytetrahydrofuran is easily manufactured in two steps from .gamma.-butyrolactone. Unfortunately, .gamma.-butyrolactone and the process to make 2-hydroxytetrahydrofuran from .gamma.-butyrolactone are actually quite expensive.
In aqueous media, 2-hydroxytetrahydrofuran and 4-hydroxybutanal are believed to equilibrate: ##STR1## These compounds are structural isomers, and are inseparable for practical purposes.
An inexpensive source of 4-hydroxybutanal is an aqueous stream of the aldehyde that is produced when allyl alcohol is hydroformylated in the presence of a rhodium catalyst. The aqueous 4-hydroxybutanal (about 12% aldehyde) is ordinarily hydrogenated to give commercially important 1,4-butanediol, which can be converted to other important compounds such as tetrahydrofuran, .gamma.-butyrolactone, and N-methyl-2-pyrrolidone.
Equilibrium complicates the synthesis of pure 2-oxytetrahydrofurans from aqueous mixtures of 4-hydroxybutanal and alcohols. When a substantial amount of water is present, the equilibrium lies to the left and favors formation of 2-hydroxytetrahydrofuran rather than the desired 2-oxytetrahydrofuran product: ##STR2## Shifting the equilibrium further to the right by distilling away water is not feasible, and is unacceptable commercially because it is energy intensive. Using an excess amount of the hydroxy compound favors the desired 2-oxytetrahydrofuran, but adds excessive cost, especially if the hydroxy compound is relatively valuable.
An economical process for producing 2-oxytetrahydrofurans is needed. A preferred process would use readily available aqueous 4-hydroxybutanal. A process that overcomes the difficulties of preparing acetals in acidic aqueous media, and permits high conversion of 4-hydroxybutanal and simple isolation of relatively pure 2-oxytetrahydrofurans is especially needed.